It is in frobt of a window and gettubg direct sunlight most if the day
I can't get an idea how much light it is getting. If that is the brightest as in the picture, it's not enough. There is a light compensation point, different for different plants. When they can't get to that point, they struggle. You'll see withering, yellowing, leggy, not growing really. I see you have valis. What happens to this plant for example when it doesn't get enough light is the top half of the leaves browns/yellows and just falls off. And the roots barely develop and you can pull the plant out easily.
Light in a low tech tank also compensates for the lower CO2 levels as plants have mechanisms to substitute resources of energy. Therefore if you can't get enough light( I am not talking extreme amount of light), you should have enough co2 and vice versa. High tech tanks get both to get the maximum growth. But having very low light and low co2 is not a successful recipe.
Good luck. I think I said all I think I should point out. Time will tell if you can modify it enough to get it going.